Aperture coupled patch antennae are known for use in receiving and transmitting high frequency signals such as microwave signals. These are particularly suitable for mobile satellite communications and are applicable to communication systems in mobile vehicles. An example of such a patch antenna is shown in U.S. Pat. No. 5,043,738. That specification illustrates an aperture coupled antenna and systems of this type are particularly applicable to use on a vehicle glazing panel as it may avoid obtrusive protrusions outside the glass surface. No electrically conducting connections are required through the glass of the glazing panel and in a preferred arrangement the patch antenna is arranged to cause only minor signal strength degradation with rain drops or water on the outside of the vehicle surface. Such systems may be used to transmit or receive plane polarised radiation or in some cases circular polarisation may be preferred. Circular polarisation is particularly suited to vehicle communication systems in that they are less sensitive to vehicle direction. Various proposals have been made for apertures and feed lines used in such coupling systems. In U.S. Pat. No. 5,043,738 there is described a system in which two separated apertures are provided in a ground plane so as to provide coupling between transmission feed circuit members and a radiating patch. To obtain circular polarisation that specification uses two laterally separated feed members having their end regions directed at right angles to each other so that each intersects the line of its respective aperture in the ground plane. However such a system requiring laterally spaced feed conductors with apertures forming separated arms of an L shaped configuration does not provide either symmetrical or strong coupling through the aperture to the radiating patch.
When used in a vehicle glazing panel it is of course necessary that the dielectric layer between the ground plane and a radiating patch will be formed of glass and this will have a thickness determined by the particular glazing requirements of the vehicle. Due to the high dielectric constant of glass and the relatively large separation between the ground plane and the radiating patch which will result from using typical thickness of toughened automotive glass, it is important to achieve an effective coupling through the aperture between the transmission feedline circuitry and the radiating patch.
It is also known to provide crossing feed members which are aligned with a cross shaped aperture but in an arrangement where the two feed members make electrical contact with each other at their cross-over point. Such a system does not provide good quality circular polarisation due to current interchange between the feed members in the region of coupling with the aperture.
It is important to use a feed system having isolated current paths in the feed system in the region of coupling through an aperture in order to achieve good quality circular polarisation of radiation from the patch.
It is an object of the present invention to provide an antenna system having improved coupling between a transmission line circuit and a radiating patch through an aperture in a ground plane, and a feed system which can generate high quality circular polarisation.
It is a further object of the invention to provide an improved antenna system suitable for use in a vehicle glazing panel.